The invention of this application relates, as indicated, to polypropylene compositions. More particularly, it relates to such compositions which are stabilized against deterioration that ordinarily results from prolonged exposure to sunlight. Still more particularly, it relates to the stabilization of polypropylene compositions by means of the combination of small proportions of additives.
Ultraviolet light has a degradative effect upon olefin polymers, the severity of which is dependent primarily on the degree of exposure. The effect is especially severe on polypropylene, probably because of the high concentration of tertiary carbon atoms which are peculiarly susceptible to oxidative attack.
Ordinarily, polypropylene is characterized by high tensile strength, i.e., higher than that of other olefin polymers, as well as by high abrasion resistance and a lustrous surface which is resistant to acid, alkali, solvent and other chemical attack. It is also very light in weight, i.e., it has a specific gravity of about 0.90. All of these physical properties, plus the fact that, as a fiber, it can be oriented so as to significantly increase its tensile strength, are highly desirable and, as expected, it finds wide usage in many applications.
Unfortunately, these desirable qualities are accompanied by a tendency of the polypropylene to deteriorate rapidly by means of photooxidation upon exposure to sunlight. The result is discoloration, reduced molecular weight and embrittlement. It is accordingly necessary to stabilize the polymer against such deterioration, and this is done by the addition of small amounts of chemical agents which are effective for this purpose. Hydroxybenzophenones, hydroxyphenylbenzotriazoles and phenylsalicylates are thus effective and are used in this manner. They operate by absorbing harmful ultraviolet energy and reemitting it at a different energy level which does not adversely affect the polymer.
The relative instability of polypropylene is due to the presence of tertiary carbon atoms in the polymer chain; these are subject to oxidative attack which is catalyzed by heat and light. In the case of fiber compositions, for example, light is an especially important factor because of the relatively large amount of surface area of the polypropylene.
Japanese Pat. No. 66551/77 (Application No. 144,357/50) shows the stabilization of certain synthetic resins, including polypropylene, by the combination of a substituted piperidine, a triorganophosphite and a secondary organophosphite. The secondary organophosphite is an essential component of the stabilizer combination.
U.S. Pat. No. 3,640,928 (Murayama et al.), which deals with the stabilization of a wide variety of synthetic polymers, shows the use of certain piperidine derivatives for that purpose. Included among such piperidine derivatives is bis(2,2,6,6-tetramethyl-4-piperidyl)adipate. It is stated that these compounds can be used alone or in combination with other known stabilizers, fillers, pigments and the like. U.S. Pat. No. 3,840,494 (Murayama et al.) shows much the same thing.
West German Pat. No. 25 00 314 deals with the stabilization of pigmented polyolefin compositions. Stabilization is accomplished by means of cyclic di-tert-alkylamines; these are defined very broadly and illustrated profusely, and include bis-(2,2,6,6-tetramethyl-4-piperidinyl-1-oxyl)-sebacate (see page 15). It is also stated that these cyclic di-tert-amines can be used for their intended purpose in combination with other known additives.